Electrophotography
The present invention is related to the photoconductor materials suitable for electrophotography. In the conventional electrophotographic process, electrostatic charge is utilized as the key component for recording information and reading out information. The recording process involves a photoconductive material that must be capable of: a) holding an electrostatic charge in darkness, and b) dissipating this electrostatic charge when exposed to a suitable light source of a wavelength that is strongly absorbed by the photoconductive material. The requirement of holding electrostatic charge can be realized if the photoconductor can exhibit a surface resistivity greater than 10.sup.13 ohm-cm in darkness, i.e. the photoconductor must be a good insulator in the dark. The requirement of releasing the electrostatic charge under light exposure is related to the significant decrease of the surface and the bulk resistivity during the process of light exposure. Thus, the requirements for the xerographic or electrophotographic photoconductor are different from that of photoconductors utilized in opto-electronic devices, such as photodiodes, solar cells, photodetectors, etc.
Electrophotographic processes have been successfully utilized in reprographic, copier, and duplicating products from low speed print-out, in the range of 1-3 pages per minute (ppm), to high speed print-out in the range of above 100 pages per minute.
Electronic Printing Using Electrophotography
Recently, electrophotography has become important in the design of electronic printers. Generally speaking, the electronic printing process utilizing electrophotography is mainly based on synchronizing of the light source, controlled by electrical signal output from a computing device such as computer. The electrical signal turns on or off the light source in order to produce many small dots, which can be developed into visible dots by electrophotographic ink or toner. The selection and collection of these dots form a halftone image.
It should be noted that the basic difference between copying machines and electronic printers, in this case, can be identified by the position at which the toner is deposited. In the copying machine, due to the reflection of the light source from the original image being copied, the toner is attached to the non-exposed area of the photoconductor, which leaves behind the light-exposed area as white background. On the other hand, in electronic printing using electrophotography, toner is attached to the light-exposed area, and thus the light source performs as a writing head or a print head.
Laser Printing Technology Components: Laser, Infrared (IR) Photoconductor
Recently, significant progress in electronic printing has been made, and solid-state opto-electronic devices such as a laser diode or a light emitting diode (LED) have become popular as the optical print head. The laser print head provides much smaller beam diameter than LED, and it is considered a key component for high resolution print-out.
Most laser printer products in the market today utilize single-beam laser scanners. These scanners typically utilize 780 nm wavelength edge-emitting laser diodes and, therefore, there is a lot of effort in development of electrophotographic photoconductors having a suitable response at 780 nm. These conventional photoconductors, typically called infrared or "IR" photoconductors, may include inorganic compounds such as amorphous silicone, dye-sensitized CdS, ZnO, TiO.sub.2 and As.sub.2 Se.sub.3. However, progress in development of organic materials has shown organic photoconductors to have some advantages over inorganic photoconductors in terms of photo-response, cost and ecological concerns.
High-resolution, High-speed Laser Printing Technology Components:
Even though the edge-emitting laser diode exhibits productivity and excellent performance in conventional laser printers products, its applications are limited in the area of higher speed and higher resolution printing. For higher speed printing above 600 DPI, for example, at 1200 DPI, 2400 DPI, or 4800 DPI, a multi-beam scanner is effective. Such multi-beam scanners use laser diodes that are surface-emitting lasers (SEL) instead of edge-emitting diodes. Thus far, the best-performing SEL is one that emits wavelengths longer than 780 nm, for example, wavelengths above 830 nm and preferably in the range of 850 nm-1000 nm.
Therefore, it is an important goal to develop an organic photoconductor (OPC) compatible with long wavelength multi-beam scanners. Such OPC's should be capable of very high speed in the wavelength range between about 850 and 1000 nm.